This solved the leaking down problem but the next day the fill valve o ring and breech seal blew as well. I will say that the #006 seals were probably in the worst condition on this gun.

Once I let it sit overnight and knew it wasn't leaking down I did some rough chrony testing with 2.8k fills. Sorry, I'm still figuring out my chrony lighting situation so I don't always get the best strings but the gun was shooting 15.89gr JSB's at a high around 900fps and stable around 885fps, easily hitting between 25-28fpe out of the box (not bad)

Moving forward, I opted to look into 2k fill tweaks for the b50 since I do not have a 4500psi tank (yet)

The pic on my b-50 has the new spring already installed (I took it apart to show the steps). Here is what the old spring looks like compared to the new one. Old on left, new on right.

Note the diameter difference. The new spring required a slight twist and it will seat perfectly in the end cap.
I actually liked the fit of the new spring compared to the old due to the fact that the old one was very loose in the cap.

Once you get the spring in the endcap it should stay put while you position it here

And then it's just assemble in reverse order.

With the new spring running the b-50 on a 2k fill my first shot was 775fps with the same 15.89gr JSB's. I had a 3rd-4th shot spike of 805fps and then things settled down nicely through 20 or so more shots. By shot 27-28 fps dropped to the 720's. For reference the 33rd shot was at 698fps, which is where I stopped shooting. Once again, sorry for the bad chrony work, I hope to update it soon. All in all I would say with the new spring the b-50 is easily capable of 25+ shots at around 20fpe on a 2k fill. No doubt more shots can be achieved with some hammer throw adjustment which I may try later. Also, I will probably replace the transfer port seal just in case I'm losing a bit of air at that location during the shot.

So I have had a little time to play with the b50. Mounted a Simmons Whitetail Mildot and got to shooting some paper. Sight in showed how important the barrel clamp is. Since I have had mine apart a couple times I had to do a preliminary sight in adjusting the barrel clamp to change POI and then fine tune with the scope adjustments. This helped to keep the scope in more or less it's middle ground of adjustment. I may add a drooper mount, not sure yet.

Other than that since I last posted I adjusted the hammer throw. The adjustment piece was 0.129" out from hammer at it's factory setting, I now have it set at 0.153". Sometime soon I will chrony to see what this adjustment did to FPS.

Anyways, I put a video together. Mind you I still have not cleaned the barrel and for some reason I'm not shooting so well today. Time will tell. Pellets used in the video are JSB 15.89.